1. Field of the Invention
This invention relates to a novel silacyclohexane compound and also to a liquid crystal composition comprising the compound and a device comprising the composition.
2. Description of the Prior Art
The liquid crystal display devices make use of optical anisotropy and dielectric anisotropy of liquid crystal substances. Depending on the mode of display, a variety of display systems are known including those of a twisted nematic type (TN type), a supertwisted nematic type (STN type), a super birefringence type (SBE type), a dynamic scattering type (DS type), a guest/host type, a type of deformation of aligned phase (DAP type), a polymer dispersion type (PD type), and an optical mode interference type (OMI type). The most popular display device is one which is based on the Schadt-Helfrich effect and has a twisted nematic structure.
Although the properties of the liquid crystal substances used in these liquid crystal devices depend, more or less, on the type of display, it is commonly required that the liquid crystal substances have a wide range of temperatures working as a liquid crystal and that they be stable against moisture, air, light, heat, electric field and the like. Moreover, the liquid crystal substances should desirably be low in viscosity and should ensure a short address time, a low threshold voltage and a high contrast in cells.
Liquid substances which can satisfy all these requirements have never been known when used as a single compound. In practice, several to ten and several liquid compounds or latent liquid crystal compounds are mixed and used in the form of a mixture. To this end, it is important that constituent components be readily compatible with one another.
Typical of such constituent components are ester compounds having a relatively high nematic-isotropic transition temperature, T.sub.NI. The ester compounds include those compounds having a phenyl ester structure of bicyclohexylcarboxylic acids of the following formulas. ##STR3## wherein R represents an alkyl group having from 1 to 8 carbon atoms as set out in Japanese Patent Publication No. 60-17777. ##STR4## wherein R represents an alkyl group having from 1 to 8 carbon atoms and R' is an alkyl group or alkanoyloxy group having less than 8 carbon atoms as set out in Japanese Patent Publication No. 60-17777. ##STR5## wherein R and R' are, respectively, an alkyl group having less than 8 carbon atoms as set out in Japanese Patent Publication No. 60-17777. ##STR6## wherein R is an alkyl group having from 1 to 15 carbon atoms and X is F, Cl or Br as set out in Japanese Patent Publication No. 61-26979. ##STR7## wherein R is a linear alkyl group having from 1 to 9 carbon atoms as set out in Japanese Patent Publication No. 62-54783. ##STR8## wherein R is an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 1-42260. ##STR9## wherein R is an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 1-50691. ##STR10## wherein R is an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 1-50694. ##STR11## wherein R is an alkyl group having from 1 to 10 carbon atoms and X and Y are independently H or CH.sub.3 as set out in Japanese Patent Publication No. 2-12211. ##STR12## wherein R is an alkyl group having less than 12 carbon atoms where one or two of the CH.sub.2 groups may be replaced by O or CH.dbd.CH as set out in Japanese Patent Publication No. 4-501275. ##STR13## wherein R is an alkyl group having from 1 to 10 carbon atoms and X is R or OR as set out in Japanese Patent Publication No. 2-10820. ##STR14## wherein R is an alkyl group having from 1 to 10 carbon atoms and X is F or Cl as set out in Japanese Patent Publication No. 2-10820. ##STR15## wherein R.sup.1 and R.sup.2 are, respectively, an alkyl group having from 1 to 10 carbon atoms as set out in Japanese Patent Publication No. 60-17777.
As the liquid crystal display devices recently have wider utility, the characteristic properties required for the liquid crystal materials become severer along with a diversity of drive systems and working modes being in progress. In particular, liquid crystal materials which meet on-vehicle needs should have a nematic phase extended to a high temperature region from the standpoint of use conditions. For the extension of the nematic phase to a high temperature region, it is sufficient to add a liquid crystal compound having a high nematic-isotropic transition temperature, T.sub.NI, as a constituent component. The known component having a high T.sub.NI value includes, for example, 4,4"-substituted terphenyl, 4,4'-substituted biphenylcyclohexane, 4,4'-substituted cyclohexanebiphenylcyclohexane. However, these compounds cause the viscosity of mixed liquid crystal to increase, thus bringing about a response speed being lowered.